This issue occurs with Lync 2010 client as well as Skype 2013 client.

When a user logs onto a computer and lync tries to pass through credentials it fails and prompts for credentials.

Under the username section you have to remove the email address and enter \username and then it works fine.

We have about 12,000 clients and we get 6-10 calls per day.

Any help would be appreciated.

Are the machines joined to a domain or are they standalone?  If it's not domain joined, this may not be a failure to pass through credentials, but instead it may just not know the username and is taking a guess based upon the supplied SIP address.  Once valid credentials are entered and it has signed in it shouldn't prompt anymore.  If your UPN matches the sip address, the problem may also disappear.

Hi mkent002,

Agree with Anthony.

If the SIP address does not match the users UserPrincipleName (UPN) in Active Directory, users will be prompted for credentials on sign-in.

The credentials format should be as shown in the screenshot below.

Best regards,

Eric

Hi mkent002,

I'm marking the reply as answer as there has been no update for a couple of days.

If you come back to find it doesn't work for you, please reply to us and unmark the answer.

Best regards,

Eric